As digital audio broadcasts for mobile bodies, DAB (Digital Audio Broadcast) systems have been introduced into actual use in recent years wherein digital audio signals are encoded with a high efficiency and modulated by OFDM (Orthogonal Frequency Division Multiplexing). Studies are also underway on IBOC (In-Band On-Channel) digital broadcasts which utilize the same frequency band as the existing analog audio broadcasts.
For the transmission of digital audio signals, studies are undertaken, for example, to allocate a digital carrier to a frequency band not in use and included within the range of regulation frame R concerning the frequencies and signal strengths assigned to the existing analog FM broadcasts as shown in FIG. 2(a). In this case, digital audio broadcast signals are transmitted in a transmission spectrum format shown in FIG. 2(b). Stated more specifically, the single bit stream providing a digital audio signal is divided into units of bits to form a plurality of (e.g., four) bit frames A, B, C and D as shown in FIG. 3. Different carrier frequencies are then allocated to the respective bit frames, which are arranged on the frequency axis as illustrated in FIG. 2(b). These bit frames A, B, C and D are transmitted cyclically with time differences.
The receiver receives the bit frames A, B, C and D and demodulates these bit frames to reproduce audio data. Processing the bit frames A, B, C and D individually for reproduction involves some degradation of audio quality, but the same contents as the original bit stream can then be retained.
In the above mode of digital audio broadcast for mobile bodies, the movement of the mobile body carrying the receiver is likely to entail an impaired reception condition. The user then needs to switch the current broadcasting station to some other station providing a better reception quality, hence a problem of cumbersome procedure.